Caption: X and Y chromosomes. Computer artwork of an X and a Y chromosome. The end regions mark out the telomeres. Telomeres consist of short repeated sequences of DNA and proteins, and are looped. The loop structure prevents the end of the chromosome being eroded by enzymes present in the cell nucleus and prevents enzymes from joining two chromosomes together. Telomeres play an important role in cellular aging. As cells replicate and age the telomere steadily shrinks. When the telomere reaches a certain length the cell stops dividing and dies.